The present invention relates to a method for producing a mother alloy for an iron-based amorphous alloy containing a sufficiently small amount of Al, which is harmful to the surface conditions and toughness of the amorphous alloy.
Amorphous alloy ribbons are widely used for cores for transformers, choke coils, etc. Nanocrystalline alloy ribbons having microstructures having crystal grain sizes of several tens of nanometers are also widely used for various magnetic parts because of their excellent soft magnetic properties. The nanocrystalline alloy ribbons are produced by heat-treating the amorphous alloy ribbons.
These magnetic alloy ribbons are generally produced by melt-quenching methods such as a single-roller method, a double-roller method, etc. and wound to form toroidal magnetic cores. The details of the melt-quenching methods are described in the chapter of xe2x80x9cMelt-Quenching Methodxe2x80x9d in Masumoto, et. al., xe2x80x9cAmorphous Alloys.xe2x80x9d For instance, the single-roller method is a method of ejecting a molten alloy onto a rapidly rotating cooling roller, so that the molten alloy is quenched to continuously form a ribbon.
Iron-based magnetic alloys having various compositions have been developed depending on their applications. Among them, iron-based magnetic alloys containing Nb are known to have excellent soft magnetic properties. For instance, Japanese Patent Publication No. 60-38454 discloses amorphous Fexe2x80x94Nbxe2x80x94Sixe2x80x94B alloys having excellent effective magnetic permeability. As described in Japanese Patent Publication No. 4-4393, nanocrystalline, magnetic Fexe2x80x94Cuxe2x80x94Nbxe2x80x94Sixe2x80x94B alloys have excellent soft magnetic properties. Nb functions as an element for decreasing a magnetostriction to improve soft magnetic properties in the amorphous alloys, and an element effective for making crystal grains finer in the nanocrystalline alloys. In these soft magnetic alloys containing Nb, high-purity Nb has conventionally been used as a raw material, because high-purity Nb contains small amounts of impurity elements such as Al harmful to magnetic properties and toughness of the amorphous alloys. However, because the high-purity Nb is extremely expensive as a raw material, it makes the production cost of alloy ribbons higher.
From the industrial point of view, ferroniobium is preferable, because it is as cheap as {fraction (1/10)} of the high-purity Nb. However, the ferroniobium contains impurity elements, particularly Al in an amount of about 0.1-2 mass %, because the ferroniobium is produced by a Thermit method. The Thermit method is a refining method using fine Al powder for reducing metal oxides such as Nb2O5. Thus, there remains a relatively large amount of Al in ferroalloys (iron alloys) such as ferroniobium produced by the Thermit method.
Al contained as an impurity is oxidized to Al2O3 during melting. In the case of producing an amorphous alloy ribbon by a single-roller method, a slag or inclusion based on Al2O3 are accumulated in a nozzle for ejecting a molten alloy, resulting in streaks on the ribbon surface in its longitudinal direction. If there were streaks on the surface of the ribbon, a toroidal coil formed from the ribbon would have a low packing factor because of surface roughness, resulting in failure to the miniaturization of the toroidal coil and extremely poor magnetic properties. Also, because Al2O3 remains as a non-metallic inclusion in the alloy ribbon, the ribbon is brittle, because the non-metallic inclusion acts as sites for fracture. If the ribbon were brittle, it would be cut when wound to a toroidal coil, resulting in extremely disturbed processes and poor yield.
Accordingly, an object of the present invention is to provide a method for producing a mother alloy suitable for iron-based amorphous alloys having a low Al content, capable of providing amorphous alloy ribbons with excellent magnetic properties free from streaks and brittleness, even when inexpensive ferroalloys such as ferroniobium containing a relatively large amount of Al are used as raw materials.
The method for producing a mother alloy for an iron-based amorphous alloy according to one embodiment of the present invention comprises the steps of (a) melting raw materials for elements constituting the amorphous alloy together with at least one oxide of an element constituting the amorphous alloy, the raw materials containing aluminum as an inevitable impurity, and the oxide having a smaller standard free energy of formation than that of Al2O3 in an absolute value; and (b) removing the resultant Al2O3 from the melt, thereby reducing the content of aluminum to 50 ppm or less in the melt.
The method for producing a mother alloy for an iron-based amorphous alloy according to another embodiment of the present invention comprises the steps of (a) melting raw materials for elements constituting the amorphous alloy to form a melt, the raw materials containing aluminum as an inevitable impurity; (b) blowing an oxygen-based gas into or onto the melt; and (c) removing the resultant Al2O3 from the melt, thereby reducing the content of aluminum to 50 ppm or less in the melt. In the method according to another embodiment of the present invention, at least one oxide of an element constituting the amorphous alloy is preferably used together with the raw materials for elements constituting the amorphous alloy, the oxide having a smaller standard free energy of formation than that of Al2O3 in an absolute value.
Ferroniobium is preferably used as one of raw materials for elements constituting the amorphous alloy. The oxide of an element constituting the amorphous alloy is preferably at least one of iron oxide, copper oxide, silicon oxide and boron oxide, more preferably iron oxide (Fe2O3).
One of the important features of the present invention is to use at least one oxide of an element constituting the amorphous alloy, which has a smaller standard free energy of formation than that of Al2O3 in an absolute value. The standard free energy of formation of a substance is defined as the free energy increase of the reaction in a standard state at 25xc2x0 C., in which the substance is formed from elements. The standard free energies of formation are described, for instance, in the column of xe2x80x9cStandard Free Energies of Formation for Oxidesxe2x80x9d in xe2x80x9cMetal Data Book,xe2x80x9d edited by the Japan Institute of Metals, and in the figures of standard free energies of formation for oxides in xe2x80x9cNew Lecture on Metallurgy, Nonferrous Metal Refiningxe2x80x9d of the Japan Institute of Metals.
For instance, when ferroalloys such as ferroniobium and ferroboron, which contain a relatively large amount of Al, are combined with Fe2O3, one of the oxides having a smaller standard free energy of formation than that of Al2O3 in an absolute value, Fe2O3 is reduced to iron, a main element constituting the iron-based amorphous alloy, by the oxidation-reduction reaction of Fe2O3+2Al=Al2O3+2Fe. On the other hand, Al contained as an inevitable impurity in the ferroalloys are oxidized to Al2O3, which floats as a slag on a surface of the resultant melt. By removing Al2O3, the melt is purified, with a reduced amount of Al harmful to the surface conditions and toughness of an amorphous alloy ribbon, which is to be formed from the mother alloy. Metal elements formed as by-products of Al2O3 can be used as elements for constituting the amorphous alloy. For instance, Fe formed from Fe2O3 by the above oxidation-reduction reaction constitutes a main element for the iron-based amorphous alloy.
Oxides usable in the present invention are iron oxide, copper oxide, silicon oxide and boron oxide. The iron oxide may be Fe2O3, FeO, Fe3O4 or mixtures thereof. The copper oxide may be CuO, Cu2O or mixtures thereof. The silicon oxide may be SiO2, and the boron oxide may be B2O3.
The amount of the oxide added to the melt may be adjusted depending on the Al contents in the raw materials. In practice, the amount of the oxide added is preferably 0.005-1 mass %, more preferably 0.01-0.5 mass % based on the melt. The oxide may be introduced simultaneously with or after adding the raw materials for amorphous alloy-constituting elements.
Another feature of the present invention is to use ferroalloys such as ferroniobium and ferroboron. In the case of an iron-based amorphous alloy containing Nb as an indispensable element, inexpensive ferroniobium may be used. A commercially available ferroniobium contains 0.1-2 mass % of aluminum. The present invention is effective particularly when such Al-containing, commercially available ferroniobium is used. Even with such inexpensive raw materials, the method of the present invention can produce amorphous alloy ribbons free from streaks and brittleness.
A further feature of the present invention is that the iron-based amorphous alloy contains 50 ppm or less of Al. When the Al content in the iron-based amorphous alloy exceeds 50 ppm, the iron-based amorphous alloy suffers from surface streaks and brittleness. Incidentally, the reduction of the Al content to less than 5 ppm results in a higher production cost. Therefore, the lower limit of the Al content in the iron-based amorphous alloy is about 5 ppm from the industrial point of view.
In another aspect of the present invention, an oxygen-based gas is blown into or onto the melt for the iron-based amorphous alloy, to form Al2O3, which should be removed. The way of blowing the oxygen-based gas is not restrictive. The oxygen-based gas may be blown from above the melt, or blown into the melt via a lance.
The oxygen-based gas comprises oxygen as a mainly component. Accordingly, it is not only pure oxygen, but also may be an oxygen-based, mixed gas containing inert gases such as argon and helium. The air may be used as the oxygen-based gas.
It should be noted that the blowing of an oxygen-based gas may be combined with the addition of at least one oxide of an element constituting the amorphous alloy, which has a smaller standard free energy of formation than that of Al2O3 in an absolute value.
The amorphous alloy ribbon may be produced by any known methods, for instance, melt-quenching methods such as a single-roller method, a double-roller method, etc. The production of the amorphous alloy ribbon may be carried out in an atmosphere of the air or an inert gas or in vacuum.
The amorphous alloy ribbon produced according to the present invention may have a thickness of about 5-100 xcexcm and a width of about 1-300 mm.
The iron-based amorphous alloys may have compositions of Fexe2x80x94Nbxe2x80x94Sixe2x80x94B, Fexe2x80x94Nixe2x80x94Nbxe2x80x94Sixe2x80x94B, etc., and the iron-based nanocrystalline alloys obtained by heat-treating the corresponding amorphous alloys at temperatures higher than their crystallization temperatures may have compositions of Fexe2x80x94Cuxe2x80x94Nbxe2x80x94Sixe2x80x94B, Fexe2x80x94Zrxe2x80x94Nbxe2x80x94B, Fexe2x80x94Nbxe2x80x94B, etc., which can provide nanocrystalline structures of several tens of nanometers in crystal grain size. Particularly in the case of the iron-based amorphous alloys containing Nb as an indispensable element, inexpensive ferroniobium containing Al as an impurity element is preferably used because Al can be removed in the form of Al2O3. In the other ferroalloys containing no Nb, such as ferroboron, the present invention is effective because Al inevitably contained therein can be removed in the form of Al2O3.
The preferred examples of the compositions of the iron-based amorphous alloys are FebalNb0.1-30Si0.1-30B1-25, FebalNi0.1-30Nb0.1-30Si0.1-30B1-25. The preferred examples of the compositions of the iron-based, nanocrystalline alloys are FebalCu0.1-0.3Nb0.1-30Si0.1-30B1-25, FebalZr0.1-30Nb0.1-30B1-25, and FebalNb0.1-30B1-25.
The present invention will be described in detail referring to EXAMPLES below without intention of limiting the present invention thereto.